This invention relates to a method and apparatus to account for a degradation of a cathode humidification unit (CHU) over a period of time and more specifically to adapting CHU model parameters to ensure an efficient operation of the CHU over the period of time.
Fuel cells, particularly proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cells, require balanced water levels to ensure proper operation. For example, it is important to avoid having too much water in the fuel cell, which can result in the flooding or related blockage of the reactant flowfield channels. On the other hand, too little hydration limits the conductivity of the ion-transmissive membrane that is disposed between catalyzed electrodes; this high ionic resistance can lead to poor electrical performance, as well as premature cell failure. One popular way to promote proper levels of humidification or related water balance within the fuel cell is through one or more CHU (also referred to as the water vapor transfer (WVT) unit, membrane humidifier, fuel cell humidifier or the like). In a typical CHU configuration, wet-side and dry-side reactant flowpaths (for example, a cathode exhaust and a cathode inlet) are in moisture-exchange communication with one another through a membrane media in the CHU such that excess moisture leaving the cathode exhaust may diffuse through the media to the drier flowpath on the cathode inlet. Examples of WVT units may be found in U.S. Pat. Nos. 7,749,661, 7,875,396 and 8,048,585, all of which are assigned to the assignee of the present invention and the entire contents of which are herein incorporated fully by reference.
In situations where numerous fuel cells are arranged as part of a module, stack or related larger assembly of fuel cell system components, a good measure of an overall humidification level for the various cell membranes can be derived from a relative humidity sensor placed in the cathode inlet gas stream. This measurement is used in conjunction with other factors, for example, cathode inlet air flowrate, cathode inlet temperature and cathode inlet pressure, to estimate the water transfer rate (WTR) of the CHU as one indicia of its performance.
There are other ways of acquiring humidity information besides using the aforementioned sensors. One way takes advantage of a fuel cell's inherent high frequency resistance (HFR), which is a directly-measurable property related to the ability of protons to pass through the cell's ion-transmissive membrane; this mobility is in turn is a function of the level of humidification of the cell. One approach to using HFR as a way to estimate and control cathode inlet and outlet flow humidities may be found in U.S. Application 2011/0113857, filed on Nov. 19, 2009 and entitled Online Estimation of Cathode Inlet and Outlet RH from Stack Average HFR, which is owned by the Assignee of the present application and incorporated herein by reference.
While determining an HFR between stack terminals may provide a good measure of average stack membrane relative humidity for helping to meet stack efficiency targets, it is not sufficient for identifying issues related to CHU degradation or wear. The conventional way of characterizing CHU degradation is to perform off-line testing of the unit while on a component test stand. This necessitates removing the CHU from the fuel cell system, testing it on the component test stand and reinstalling the unit back in the system; such an approach requires a lot of CHU downtime (for example, about 48 hours). Consequently, performing frequent off-line testing of fuel cell systems—such as those contemplated for vehicular applications—as a way to determine unit degradation is not practical.
The CHU model takes the operating conditions, estimates the water transferred from the cathode outlet to the cathode inlet and then reports the stack cathode inlet molar water molar flowrate. The inlet water flowrate is used in the water buffer model to determine the outlet conditions of the stack cathode, which are then sent to the CHU wet side inlet. The inlet of the CHU is modified to account for water transfer to the anode side of the stack, which reduces the amount of water recycled to the CHU.
Over time, the CHU degrades causing the current humidification algorithms to overestimate the amount of water transferred back to the inlet of the fuel cell stack. This overestimate will affect the outlet humidification more at low current density than high because of the higher cathode stoichiometries run at low power. Higher stoichiometry means that the product water, an easily calculated number, is a smaller fraction of the total stack outlet water and that the inlet water, a quantity dependent on the CHU's performance, is a much larger fraction. As the CHU degrades, there is lower inlet water and therefore a drier membrane. A dry membrane produces a chemically induced degradation of the inlet portion of the fuel cell. This will increase the HFR measurement and result in lower cell voltage and lower system efficiency. Alternatively the operating conditions can be adjusted to increase cell humidification (lower temp, lower stoich, increased pressure for example), but all of these have negative impacts such as moving off the most efficient operating point, or limited heat rejection. Furthermore, as the CHU degrades, the fuel cell stack will be drier when operating temperatures are high, potentially leading to unacceptable system performance and material degradation.
Therefore a need exists to account for the degradation over the life of the CHU.